Sensor and method of operating the sensor

ABSTRACT

The invention relates to a sensor and a method of operating a sensor with includes a plurality of sensor elements ( 10 ), each of which includes a radiation-sensitive conversion element ( 1 ) which generates an electric signal in dependence on the incident radiation, and also with means ( 21  to  26 ) for amplifying the electric signal in each sensor element ( 10 ) and a read-out switching element ( 30 ) in each sensor element ( 10 ) which is connected to a read-out line ( 8 ) in order to read-out the electric signal. In order to provide a sensor in which a high stability of the transfer function and a favorable signal-to-noise ratio are ensured while maintaining a comparatively simple and economical construction, the means for amplifying include a respective source follower transistor ( 21 ) whose gate is connected to the conversion element ( 1 ), whose source is connected on the one side to an active load ( 23 ) and on the other side to one side of a sampling capacitor ( 26 ), the other side of the sampling capacitor ( 26 ) being connected to the read-out line ( 8 ) via the read-out switching element ( 30 ), a respective reset element ( 27 ) being connected to the conversion element ( 1 ) so as to reset the conversion element ( 1 ) to an initial state.

[0001] The invention relates to a sensor with a plurality of sensorelements, each of which includes a radiation-sensitive conversionelement which generates an electric signal in dependence on the incidentradiation, and also with means for amplifying the electric signal ineach sensor element and a read-out switching element in each sensorelement which is connected to a read-out line in order to read out theelectric signal. The invention also relates to a method of operatingsuch a sensor as well as to an X-ray examination apparatus whichincludes an X-ray source for emitting an X-ray beam for irradiating anobject so as to form an X-ray image, as well as a detector forgenerating an electric image signal from said X-ray image.

[0002] Large-surface X-ray detectors are customarily used for X-rayexamination applications, notably in the medical field; such detectorsconsist of a plurality of sensor elements. The sensor elements (pixels)as a rule are arranged in rows and columns in a sensor matrix.Preferably, use is made of the so-called flat dynamic X-ray detectors(FDXD). Such detectors are seen as universal detector components thatcan be used in a wide variety of X-ray apparatus.

[0003] In contemporary FDXD embodiments, the individual sensor elements(matrix cells) comprise a radiation-sensitive conversion element, havingan intrinsic storage capacity, and a switching element for reading outthe signal present on the conversion element or the storage capacitanceafter the irradiation. The FDXD preferably utilize conversion elementsin the form of photodiodes of amorphous silicon and scintillatorelements connected thereto, or alternatively photoconductors, for thedirect conversion of the X-rays into electric charges. In other types ofsensors for other radiation, of course, other conversion elements canalso be used.

[0004] Diode switches or transistors, notably TFTs (thin filmtransistors) of amorphous silicon are preferably used as read-outswitching elements. In order to read out the signal, present as acollected charge on the conversion element or the intrinsic storagecapacitance thereof, the read-out switching elements are activated andthe collected charge is conducted to the relevant read-out line. Fromthere it flows to a charge-sensitive amplifier (CSA). Subsequently,corresponding electronic information is applied to a multiplexer whichconducts this information to a data acquisition unit for display on adisplay device in the form of a monitor.

[0005] When such detectors are used, notably in the medical analysispractice, it is desirable to reduce the radiation dose so as to limitthe dose whereto the patient is exposed; consequently, only a very smallamount of radiation is incident on the individual sensor elements. As aresult, the electric signal in the individual sensor elements is alsovery small. Therefore, the aim is to realize sensors or X-ray detectorshaving an as high as possible signal-to-noise ratio.

[0006] A particularly high signal-to-noise ratio and detection of smalldoses, of course, is also desirable for other radiation-sensitivesensors. In order to improve the signal-to-noise ratio, the signal canin principle be amplified already in the individual matrix cell of thedetector.

[0007] U.S. Pat. No. 5,825,033 discloses a semiconductor detector forgamma rays in which the charge generated in each pixel in the detectormaterial is stored in an integration capacitor of a capacitive feedbackamplifier. This integration takes place for all pixels simultaneously.In a so-called Correlated Double Sample-and-Hold circuit (CDSH) thenoise induced by the resetting of the integration capacitor iseliminated. Subsequent to the CDSH, the individual pixels are connectedto a respective unity gain buffer which is connected to a read-out linecommon to each column. The read-out lines are then combined byappropriate multiplexers. The sensor in this case consists of a matrixwith 48×48 individual pixels.

[0008] For amplifier circuits for enhancing the signal-to-noise ratio,the signal amplification and the noise are customarily the essentialcharacteristics considered for evaluation. For practical operation thereis a further criterion in the form of the stability of the transferfunction. For example, when the signal amplification or an offset valueof the amplifier fluctuates in time, offset and gain artefacts occur inthe imaging detector system; such artefacts can only be corrected partlyand with great effort only. Such fluctuations may be caused by changesof the temperature or other operating conditions as well as be due toaging, radiation damage and/or trapping effects in semiconductors.

[0009] The threshold voltage and the transconductance are liable tochange significantly in time, notably in the frequently used thin filmtransistors (TFTs) of amorphous silicon, which can also be used notablyfor the manufacture of integrated amplifier circuits in a matrix cell;this may degrade the stability of the transfer function.

[0010] Therefore, it is an object of the present invention to provide asensor and a method of operating the sensor wherein a high stability ofthe transfer function and an attractive signal-to-noise ratio areensured by a comparatively simple and economical construction.

[0011] This object is achieved by means of a sensor which ischaracterized in that the means for amplifying include a respectivesource follower transistor whose gate is connected to the conversionelement, whose source is connected an active load and to one side of asampling capacitor, the other side of the sampling capacitor beingconnected to the read-out line via the read-out switching element, andthat a respective reset element is connected to the conversion elementin order to reset the conversion element to an initial state.

[0012] The active load ideally constitutes a current source whichimpresses a constant channel current on the source follower transistor.The threshold voltage of the source follower transistor is thusstabilized; this threshold voltage is strongly dependent on the channelcurrent, notably in the case of TFTs of amorphous silicon. As a resultof the stable threshold voltage, the condition for correct operation ofthe source follower transistor with adequate stability of the transferfunction is satisfied. Therefore, the source follower transistor has astable voltage amplification of 1. It is converted into a chargeamplification G_(Q)=C_(S)/C_(P) by the sampling capacitor, wherein C_(P)is the capacitance on the conversion element and C_(S) is thecapacitance of the sampling capacitor. The capacitance on the conversionelement may again be an intrinsic storage capacitance of the conversionelement or an additional capacitance.

[0013] Preferably, the active load, the read-out switching element andthe reset element are also formed by transistors. All componentsrequired for the invention can then be integrated directly in the sensorelements while using the thin film technology which is used any way toform the sensor elements; in the context of this technology thetransistors can be made of amorphous silicon or polycrystalline silicon.Because of the stable amplification circuit constructed in conformitywith the invention, the use of the TFT transistors of amorphous siliconthat can be economically manufactured is not a drawback.

[0014] A process with vertical integration can now be advantageouslyused in such a manner that the surface area of the conversion element,or the storage capacitance within a sensor element, is not reduced.

[0015] In one embodiment a discharge switching element, preferably inthe form of a transistor, for example a TFT of amorphous orpolycrystalline silicon, is connected parallel to the samplingcapacitor. This discharge switching element can be used for thesimultaneous, accelerated discharging of the sampling capacitor during areset of the conversion element by means of the reset element, so thatthe sampling capacitor is also reset to an initial state.

[0016] The reset element and the discharge switching element may thenhave a common switching line so that they are always activatedsimultaneously. However, they may alternatively have separate switchinglines, so that the reset element and the discharge switching element canbe individually activated, for example for given modes of operation.Preferably, a plurality of sensor elements, for example all sensorelements of a row of the sensor matrix, have a common switching line forthe activation of the read-out switching elements. Such sensor elements,connected to a common switching line, can also have common switchinglines, or a common switching line for both elements, in order toactivate the reset elements or the discharge switching elements.

[0017] According to a particularly advantageous method of operating asensor according to the invention the conversion element and thesampling capacitor are reset to an initial state during a measuring andread-out cycle in each sensor element in a first phase. In a secondphase a voltage difference which is representative of the conversionelement in the initial state is then adjusted across the samplingcapacitor. During a third phase the voltage across the samplingcapacitor is sustained during irradiation of the conversion element bymeans of a radiation source whereas the voltage at the source output ofthe source follower is forced to change by the change of the signal atthe conversion element or of its capacitance. Evidently, in this contextthe term irradiation by means of the radiation source is to beunderstood to mean not only direct irradiation by the radiation source,but also indirect irradiation, for example after transmission through anobject to be examined. During a fourth phase the voltage differenceacross the sampling capacitor is adjusted to a value which isrepresentative of the conversion element after the irradiation, thevariation of the potential at the side of the sampling capacitor whichis connected to the read-out line then being measured as a measure ofthe radiation incident on the conversion element. Preferably, thevariation of the charge at the read-out side of the sampling capacitoris then recorded in a charge-sensitive amplifier (CSA). This means thatthe amount of charge flowing during the adjustment of the new voltagedifference is integrated.

[0018] As a result of this switching sequence a so-called “correlateddouble sampling” (CDS) method is implemented in the relevant sensorelement. This means that during the second phase a first sampling valueis detected for the conversion element in the stationary state whereasduring the fourth phase ultimately a value is measured across thesampling capacity which corresponds to the conversion element after theirradiation, only the difference between the initial state and theirradiated state being measured during the first sampling because of thebias in the second phase.

[0019] This switching process also offers the advantage that the resetoperation during the first phase lies outside the time interval in whichthe conversion element is irradiated and the signal is read out, so thatthe reset operation has no effect on the measuring result and hencecannot contribute to the noise.

[0020] According to a second version of the method of the invention, adark current is first detected on the conversion element during a firstsub-phase of the second phase, the voltage difference across thesampling capacitor being held during a given time interval withoutirradiation of the conversion element by the radiation source while atthe same time the voltage on the source output varies in conformity withthe dark current occurring across the conversion element. The darkcurrent can then be attributed essentially to leakage currents on theconversion element. This sub-phase is succeeded by a second sub-phaseduring which a voltage difference is adjusted across the samplingcapacitor, which voltage difference corresponds to a reference state ofthe conversion element after the detection of the dark current. Thesecond sub-phase is succeeded by a third sub-phase in which theconversion element is reset to its initial state, the voltage differenceacross the sampling capacitor then being maintained. The execution ofthe other phases is the same as in the previously described method.

[0021] The difference between this method and the previously mentionedmode of operation thus consists in that during the first samplingoperation the initial state, that is, the off-load voltage on theconversion element, is not taken as the reference value, but thereference state already contains the integrated dark current. This meansthat the dark images are already buffered in the individual sensorelements and subtracted from the exposed images. Thus, the transfer andexternal storage of the dark images is dispensed with. Additionally, theusable dynamic range of the sensor is expanded, since the chargestransferred from the individual sensor elements no longer contain a darkcurrent component.

[0022] The adjustment of the voltage difference across the samplingcapacitor during the second and the fourth phase is performed mosteasily by activation of the read-out switching element, that is, via theread-out line. In order to sustain the voltage difference during thethird phase, or during the dark current measurement, the read-outswitching element need only be deactivated.

[0023] The resetting of the sampling capacitor in one embodiment of theinvention can be realized by activation of the discharge switchingelement connected parallel to the sampling capacitor, thus enablingaccelerated resetting.

[0024] The measuring and read-out cycles can be controlled in common foreach time a plurality of sensor elements and via common switching lines.That is, after the irradiation the sensor elements in a sensor matrixare successively read out in rows and are reset.

[0025] According to a third version of the method of the invention,there is provided a method of operating a sensor having a plurality ofsensor elements (10) arranged in rows and columns, each of whichincludes a radiation-sensitive conversion element (1) which generates anelectric signal in dependence on the incident radiation, a reset element(27) which resets the conversion element (1) to an initial state, and asource follower transistor (21) whose source is connected to an activeload (23) and to one side of a sampling capacitor (26) whose other sideis connected, via a read-out switching element (30), to a read-out line(8) for reading out the electric signal, the method comprising:

[0026] resetting the radiation-sensitive element and charging thesampling capacitor of each pixel to a known voltage;

[0027] exposing the sensor to radiation, the radiation-sensitiveconversion element causing the voltage on the one side of the samplingcapacitor to vary, wherein the read-out switching element is open duringthe exposure, providing an open circuit at the other side of thesampling capacitor, thereby maintaining a constant charge on thesampling capacitor; and

[0028] closing the read out switching elements and charging the samplingcapacitor for each pixel in a row to the voltage on the one side of thesampling capacitor, the amount of charge required being measured.

[0029] By separating the pixel resetting from the array readout phase,this scheme provides sufficient time for the sampling capacitor to reacha steady state which eliminates the pixel offset error charge.Furthermore, the pixel readout time can be increased.

[0030] According to a fourth version of the method of the invention,there is provided a method of operating a sensor having a plurality ofsensor elements (10) arranged in rows and columns, each of whichincludes a radiation-sensitive conversion element (1) which generates anelectric signal in dependence on the incident radiation, a reset element(27) which resets the conversion element (1) to an initial state, and asource follower transistor (21) whose source is connected to an activeload (23) and to one side of a sampling capacitor (26) whose other sideis connected, via a read-out switching element (30), to a read-out line(8) for reading out the electric signal, the method comprising:

[0031] exposing the sensor to radiation with the read-out switchingelements closed, the radiation-sensitive conversion element causing achange in the voltage on the one side of the sampling capacitor and theread out line holding the other side of the sampling capacitor to aconstant voltage;

[0032] closing the reset elements to and opening the read out switchingelements, thereby holding the conversion element to a constant stateirrespective of the incident radiation;

[0033] closing the read out switching elements of each row in turn andmeasuring the charge stored on the sampling capacitors for each row inturn.

[0034] Since the reset switches remain on during the readout period, thephotodiode charge remains constant. Therefore, radiation incident on thedetector after the exposure time will not alter the signal being readoutthrough the sampling capacitors so that Frame Transfer operation ispossible.

[0035] An X-ray examination apparatus according to the inventionincludes an X-ray source for emitting an X-ray beam for irradiating anobject so as to form an X-ray image, as well as a detector for formingan electric image signal from said X-ray image, the X-ray detector beingequipped with a sensor according to the invention. Such an X-rayexamination apparatus has a particularly attractive signal-to-noiseratio and, therefore, is capable of operating with small doses, so thatthe radiation load for the object, notably a patient, can be kept small.

[0036] Further details and advantages of the invention are disclosed inthe dependent claims and the following description in which theembodiments of the invention as shown in the Figures are described indetail. In the Figures:

[0037]FIG. 1 shows a circuit diagram of a sensor element of a sensormatrix according to the invention;

[0038]FIG. 2 shows the circuit diagram of FIG. 1 with the switching andsupply lines leading tot he components in one embodiment;

[0039]FIG. 3 shows diagrammatically a switching sequence during ameasuring and read-out cycle in conformity with a first mode ofoperation;

[0040]FIG. 4 shows diagrammatically a switching sequence during ameasuring and read-out cycle in conformity with a second mode ofoperation;

[0041]FIG. 5 shows diagrammatically a switching sequence during ameasuring and read-out cycle in conformity with a third mode ofoperation; and

[0042]FIG. 6 shows diagrammatically a switching sequence during ameasuring and read-out cycle in conformity with a fourth mode ofoperation.

[0043]FIG. 1 shows a sensor element 10 in the form of a conventionalFDXD matrix cell 10 extended with the circuit according to theinvention. Hundreds or thousands of such matrix cells 10 are arranged inrows and columns within a sensor.

[0044] Each matrix cell 10 includes first of all, as in customary FDXDmatrix cells, a conversion element 1 with a storage capacitance 2 whichmay be intrinsically contained in the conversion element 1 oradditionally built in.

[0045] The conversion element 1 and the storage capacitance 2 areconnected on one side to a counter electrode 9 which is common to allmatrix cells 10. Furthermore, each matrix cell 10 includes a read-outswitching transistor 30 whose gate is connected to a switching line 7.The switching lines 7 are common to all matrix cells 10 of a matrix row.The output of the switching transistors 30 is connected to a read-outline 8, the matrix cells 10 of a column in the customary matrixdetectors being provided with a respective common read-out line 8.

[0046] The matrix cells 10 are row-wise activated for reading out, viathe switching lines 7, so that the individual matrix cells 10 of therelevant column are read out successively via each time the sameread-out line 8. At the end of the read-out line 8 there is provided acharge-sensitive amplifier (CSA) 11.

[0047] In the conventional FDXD matrix cells known thus far theconversion element 1, or the storage capacitance 2, is connecteddirectly to the input of the switching transistor 30. This means that noamplification takes place within the individual matrix elements.

[0048] In the sensor according to the invention the side of theconversion element 1, or the storage capacitance 2, which faces thecounter electrode 9 is connected first of all to the gate of a sourcefollower transistor 21.

[0049] At the source output of the source follower transistor 21 thereis provided an additional transistor 23 which serves as an active load.Moreover, the output of the source follower transistor is connected to asampling capacitor 26, the other side of which is connected to the inputof the read-out switching transistor 30. The drain terminal 22 of thesource follower transistor 21 may be common to all matrix cells 10 andbe connected, for example, to the counter electrode 9. However, it mayalso be connected parallel to the switching line 7 so as to behorizontally common to all matrix cells 10 of a row.

[0050] Similarly, the gate terminal 24 and the source terminal 25 of theactive load 23 may be common to all matrix cells 10 of a sensor orcommon to one row only. It is in principle also possible to connect thegate terminal 24 directly to the drain of the active load 23.

[0051] The source follower transistor 21 as well as the active load 23should preferably operate in the saturation range of the relevanttransistor characteristic, that is, the condition V_(DS)>V_(GS)−V_(t)must be satisfied, where V_(DS) is the drain source voltage, V_(GS) isthe gate source voltage and V_(t) is the actual threshold voltage of therelevant transistor. The voltage transfer of the source followertransistor can then be described by the equation V_(S)=V_(G)−V_(thr),where V_(thr) is the effective threshold voltage which is dependent onthe actual threshold voltage V_(t) and the drain current I_(D). V_(S) isthe voltage present at the source and V_(G) is the voltage present atthe gate of the source follower transistor 21.

[0052] A reset transistor 27 is connected to the output of theconversion element 1, or the storage capacitance 2, which faces thecounter electrode 9; there is a reset transistor 27 serves to bias theconversion element 1 and the parallel storage capacitance 2 to theoff-load voltage V_(G0). The source terminal 29 of the reset transistors29 can be constructed so as to be common to all matrix cells 10 of thesensor. It is also possible to form the output 29 for all matrix cells10 of a row, that is parallel to the switching line 7. The gate terminal28 of the reset transistor 27 is preferably constructed so as to becommon to all matrix cells 10 of a row.

[0053] Furthermore, the circuit also includes an optional dischargingtransistor 31 whose gate 32 is also connected, preferably via ahorizontal line, so as to be common all matrix cells 10 of a row.

[0054]FIG. 2 shows, by way of example, a circuit arrangement with fouradditional horizontal lines 3, 4, 5, 6, that is, lines which extendparallel to the switching line 7 and are common to all matrix cells 10of a matrix row. A switching line 3 is connected to the gate of theactive load 23. A further switching line 4 is connected to the gate 28of the reset transistor 27 and the gate 32 of the discharging transistor31. A third line 5 is connected to the source output 29 of the resettransistor 27 and a fourth line 6 is connected to the source output 25of the active load 23.

[0055] All components shown are integrated in the matrix cells 10 by wayof thin film technology. The transistors are made of amorphous siliconor polycrystalline silicon.

[0056] Various preferred versions for operation of the proposed circuitwill be described in detail hereinafter. To this end, reference is madeto the respective switching sequences which are diagrammatically shownin FIGS. 3 to 6. The method shown in FIG. 4 utilizes double samplingwhile that shown in FIG. 3 utilizes correlated double sampling (CDS)within the relevant matrix cell 10. FIG. 5 shows a different operatingscheme which ensures the sampling capacitor is fully prepared beforeread out of pixel data and FIG. 6 shows a method which enables FrameTransfer detector operation.

[0057]FIG. 3 shows the mode of operation called “switching noisesuppression”. The top plot in FIG. 3 indicates the X-ray exposure time.The second plot shows when the read out switching transistor 30 is on oroff. The third plot shows when the reset transistor 27 and thedischarging transistor 31 are on and off, and the bottom plot shows whenthe amplifier 11 is active. During a first phase I, that is, the resetphase, the reset transistor 27 is active in the matrix cells 10 of therelevant row. As a result, the conversion element 1 and the parallelstorage capacitance 2 of the magnitude C_(P) are biased to the off-loadvoltage. The voltage V_(G0) is then present at the gate of the sourcefollower transistor 21.

[0058] In as far as the circuit does not include the optionaldischarging transistor 31, the read-out switching transistor 30 remainsactive during the entire reset phase I (solid line). In the embodimentwhich includes the optional discharging transistor 31 as shown in FIG. 1and FIG. 2, the discharging transistor 31 is active simultaneously withthe reset transistor 27 in the reset phase I so as to achieveaccelerated discharging of the sampling capacitor 26 having thecapacitance C_(S). The read-out switching transistor 30 is preferablydeactivated during the reset phase I (dotted line).

[0059] After the end of the first phase I, the read-out switchingtransistor 30 is active until an instant A in a second phase II. Duringthis time the voltage V_(G0)−V_(thr) is present at one side of thesampling capacitor 26 whereas at the other side the input voltageV_(CSA) of the CSA 11 arises via the lowpass filter formed by thesampling capacitor 26 and the read-out switching transistor 30. The CSA11 must be constructed in such a manner that its input voltage is alwaysmaintained at the constant value V_(CSA), irrespective of the factwhether the integrator in the CSA is active or not. Customary CSAssatisfy this requirement. Thus, the voltage differenceV_(G0)−V_(thr)−V_(CSA) is maintained across the sampling capacitor 26 asfrom the opening of the read-out switching transistor 30 at the instantA; this voltage difference is representative of the reset conversionelement 1. A “zero value” is thus quasi sampled.

[0060] The described reset operation in the first phase and the samplingof the relevant zero value in the second phase II are performed row-wisefor all matrix cells 10 of the detector matrix. Subsequently, in theso-called X-ray window the entire detector matrix is exposed to X-raysduring the third phase III. The charge carrier pairs then generated inthe conversion element 1 of the relevant matrix cell 10 discharge thestorage capacitance 2 of the magnitude C_(P) by the signal charge Q_(P),with the result that the voltage at the gate of the source followertransistor 21 increases to V_(G1)=V_(G0)+(Q_(P)/C_(P)). The voltageV_(G1)−V_(thr) then arises at the output of the source followertransistor 21, without the voltage difference across the samplingcapacitor 26 being changed, because the read-out switching transistor 30and the discharging transistor 31 are inactive.

[0061] During the subsequent fourth phase IV of the row-wise red-outoperation, first the integrator of the CSA 11 is activated and brieflythereafter the read-out switching transistor 30 for each matrix row.Whereas the output of the source follower transistor 21 still carriesthe voltage V_(G1)−V_(thr), the other side of the sampling capacitor 26carries the input voltage of the CSA 11 again. At the instant B theintegration in the CSA 11 is stopped. The voltage difference across thesampling capacitor 26 then amounts to V_(G1)−V_(thr)−V_(CSA). Comparisonwith the voltage difference at the instant A reveals that the samplingcapacitor 26 has been subject to a change of charge amounting toQ_(S)=C_(S)*(V_(G1)−V_(G0))=C_(S)*Q_(P)/C_(P) during the integrationtime, that is, precisely only during this time. Therefore, exactly thischarge Q_(S) is measured as the result of the integration in the CSA 11.It is advantageous that the charge Q_(S) exceeds the change of the Q_(P)of the storage capacitance 2 by the charge amplification factorG_(Q)=C_(S)/C_(P). At the instant B an operation cycle of the matrixcell 10 is terminated and the described first phase I can commenceagain. This is shown in FIG. 3.

[0062] The described mode of operation is also compatible with acontinuous X-ray exposure mode. For reasons of clarity, however, pulsedX-ray exposure was chosen. The leakage currents which practically alwaysflow in the conversion elements 1 have also been ignored for the purposeof simplicity. As in conventional FDXDs, the leakage currents in thismode of operation are contained in the measured charge signal. Whenphotodiodes are used as the conversion element 1, it is to be noted thatthe capacitance C_(P) is not constant but dependent on the charge Q_(P),so that the transfer function contains a non-linear component.

[0063] The proposed solution has a particularly advantageous aspectwhich is formed by the stability of the transfer function of thecircuit. This gain stability of the circuit is due to the fact that thesource follower transistor 21 has a stable voltage amplificationamounting to 1 which is converted into a charge amplificationG_(Q)=C_(S)/C_(P) by means of the sampling capacitor 26. The offsetstability is obtained by subtraction of the relevant offset value fromthe overall value consisting of the signal and the offset value. As aresult, all offset effects which are slower in time than the imagerepetition time T_(F) are effectively eliminated. Due to the 1/F noiseof the source follower transistors 21 and the active loads 23 used inthe proposed circuit, additional noise may occur in this mode ofoperation. However, noise phenomena which are essentially slower thanthe image repetition rate T_(F) are again eliminated by the CDS method.

[0064] As regards the switching noise it is to be noted that themeasuring result is affected only by switching operations between andincluding the instants A and B. The reset operation for the conversionelement 1 and the sampling capacitor 26 does not lie within this timeinterval and hence does not contribute to the noise. The opening of theread-out switching transistor 30 at the instant A makes a noisecontribution which is a factor G_(Q) ^(1/2) larger than the switchingnoise of the known FDXD. However, this is opposed by the signalamplification factor G_(Q), SO that overall the signal-to-noise ratio isimproved by G_(Q) ^(1/2). The switching noise upon deactivation of theintegration in the CSA 11 at the instant B does not make an additionalcontribution, because it also occurs in conventional FDXD and in thiscase loses significance in comparison with the signal because of thecharge amplification G_(Q).

[0065] Overall, this mode of operation leads to an enhancedsignal-to-noise ratio when the charge amplification G_(Q) issufficiently high; this leads to a distinct enhancement of the image,notably in the case of X-ray exposures with a low dose (for example,fluoroscopy).

[0066] The switching sequence for the second mode of operation, that is,the so-called dark current subtraction mode, is diagrammatically shownin FIG. 4. The top plot in FIG. 4 indicates the X-ray exposure time. Thesecond plot shows when the read out switching transistor 30 is on oroff. The third plot shows when the reset transistor 27 is on or off. Thefourth plot shows when the discharging transistor 31 is on and off, andthe bottom plot shows when the amplifier 11 is active. Like in the firstmode of operation, in the first phase I first the conversion element 1,or the capacitance 2, and the sampling capacitor 26 are biased to theoff-load voltage.

[0067] Subsequently, however, this value is not retained directly as thezero value on the sampling capacitor 26, but first a dark current isrecorded on the conversion element 1 in a first sub-phase IIa. The darkcurrent of the relevant conversion element 1 is then integrated.

[0068] Subsequently, in a second sub-phase IIb a voltage difference isadjusted across the sampling capacitor 26, that is, the dark image issampled. This is realized by briefly activating of the read-outswitching transistor 30.

[0069] After this first sampling in the sub-phase IIb, in the sub-phaseIIc the conversion element 1 is reset, in this case neither thedischarging transistor 31 nor the read-out switching transistor 30 isactivated, so that the voltage difference adjusted across the samplingcapacitor 26 during the first sampling operation is retained. When adischarging transistor 31 is used, of course, it must then be possibleto switch the discharging transistor 31 and the reset transistor 27 viaseparate lines.

[0070] After such second resetting of the conversion element 1, theX-ray exposure takes place in the X-ray window. Reading out in the phaseIV takes place like in the previously described “switching noisesuppression” mode of operation.

[0071] Overall, according to this method the measured value obtained atthe instant B after the integration in the CSA 11 is the chargeG_(S)=G_(Q) * (Q_(P)−Q_(D)) with the charge amplification factorG_(Q)=C_(S)/C_(P) as in the first mode of operation. The charge Q_(D)represents the dark current component integrated in the dark window.Therefore, the lengths of the dark window and the X-ray window, that is,the phases IIa and III, are preferably chosen to be the same, so that ameasured value which has been corrected in respect of the dark current.The quantities V_(thr) and V_(CSA) no longer occur in the measuredvalue, like in the previously described mode of operation, because theyare also eliminated by the subtraction.

[0072] Overall, the advantage of the second mode of operation resides inthe fact that the dark images which are produced mainly by leakagecurrents in the conversion elements 1 are subtracted already within theindividual matrix cells 10. Furthermore, this second mode of operationalso offers the advantages of a particularly advantageous stability ofthe transfer function of the circuit, that is, the gain stability andthe offset stability as already achieved for the first mode ofoperation.

[0073] Because resetting of the conversion element 1 and the storagecapacitance 2 takes place between the instants A and B within the thirdsub-phase IIc of the phase II, an additional noise component which isdue to the reset noise must be taken into account. Therefore, thesignal-to-noise ratio in the second mode of operation will be less thanthat in the first mode of operation.

[0074] The two modes described above each provide a reset operation ofthe conversion element 1 (phase I), followed by storage of a charge onthe sampling capacitor 28 corresponding to the reset state of theconversion element 1 (phase II). This is repeated for each row.

[0075] A potential problem with this approach is that the pixel must bereadout, reset and then the sampling capacitor 28 must be charged backto a steady state all within a short interval of about 20 μs. Assuming10 μs is required for readout, then this leaves 5 μs each for the othertwo tasks. Using typical device parameters for a-Si and poly-Si TFTsthere may be insufficient time for the sampling capacitor to be chargedback to a steady state within 5 μs. Ideally, a time period of 50-100 μsis appropriate.

[0076] If the sampling capacitor 28 is not recharged to a steady state,charge offsets from the pixel are caused.

[0077]FIG. 5 is used to illustrate an alternative timing scheme in whichthe resetting operation is not carried out during the read out of thearray. Instead, all pixels in the array are reset (photodiodes reset andsampling capacitor charged to steady state) in parallel before the X-rayexposure. A 2 ms time interval is allocated for this so that thesampling capacitor can be easily charged to a steady state.

[0078] For illustration only, the timing sequence of FIG. 5 assumes 30KHz operation for a detector with a 1000×1000 array of pixels. Thisgives a frame time 50 of 33 ms. This may be divided in to an X-rayexposure time of 13 ms and a line readout time of 20 μs.

[0079] The array readout is divided into three phases I, II and III,discussed below. The top plot in FIG. 5 shows the X-ray exposure time.The next plot shows the read out pulse for the first row, and the nextplot shows the read out pulse for the last row. The fourth plot showsthe state of the read out switching transistor 30 and the bottom plotshows the state of the reset transistor 27.

[0080] Phase I

[0081] This is the 2 ms reset phase during which the sampling capacitoris charged to the steady state value. At the start of the reset stageall of the reset transistors 27 are closed. The photodiode charge isreset and the gate of the source follower transistor 21 is fixed at theV_(G0) DC potential. The source of the source follower transistor 21will reach the steady state voltage V_(G0)−V_(thr). Next, the resettransistors 27 open and the read out switching transistors 30 close. Thevoltage on one plate (the top plate) of the sampling capacitor 26 isfixed at the source voltage and other plate voltage is set to the columnpotential on the read out line 8. Therefore, the charge on the samplingcapacitor 26 is constant at the end of the reset phase. Assuming thatthe column potential is OV, then the charge on capacitor 26 can beexpressed as:

Q ₀ =C _(S) ×V _(SQ)

[0082] where C_(S) is the capacitance of the sampling capacitor, andV_(SQ) is the initial (quiescent) value of the source voltage of thesource follower transistor 21 (equal to V_(G0)−V_(thr)).

[0083] Phase II

[0084] The signal exposure window follows the resetting stage. Duringthis time, photons incident onto the photodiode will generate aphotocurrent to discharge the photodiode capacitance. This results in alinear increase in the gate voltage. As described above, the active loadensures that the source voltage follows the gate voltage to maintain aconstant gate-source voltage. If the change in gate voltage during theexposure was ΔV_(pd), then the final value of source voltage V_(S1) willbe:

V _(S1) =V _(S0) +ΔV _(pd)

[0085] During the exposure time, the charge on capacitor 26 remainsconstant since the read out switching transistor 30 is open

[0086] Phase III

[0087] The final stage in the readout sequence is the line by linereadout. During readout, the read out switching transistors 30 of allpixels in a row are closed and the sampling capacitor 26 is charged tothe new value of the source voltage. The pulses may last 18 μs. Thecharge on the sampling capacitor at the end of the readout will be:

Q ₁ =C _(S) ×V _(S1) =C _(S)×(V _(S0) +ΔV _(pd))

[0088] During the readout period, the amplifier samples the change incharge across capacitor 26 as given below:

ΔQ _(C) =Q ₁ −Q ₀ =C _(S) ×ΔV _(pd)

[0089] This can be rewritten as:${\Delta \quad Q_{C}} = {\frac{C_{S}}{C_{P}} \times \Delta \quad Q_{pd}}$

[0090] where ΔQ_(pd) is the change in photodiode charge during anexposure, and the term C_(S)/C_(P) is the pixel gain.

[0091] By separating the pixel resetting from the array readout phase,this scheme provides sufficient time for the sampling capacitor to reacha steady state which eliminates the pixel offset error charge.Furthermore, the pixel readout time can almost be doubled from forexample 10 μs to 18 μs and this increases the time to read the pixelsignal.

[0092]FIG. 6 shows a further drive scheme that provides Frame TransferOperation. There is a desire to find a solution for operating X-raydetectors in bi-plane cardio applications. Such applications uses twodetectors orthogonally positioned and two X-ray sources operating atfast frame rates (60Hz). The X-ray sources are pulsed sequentially sothat a first detector detects a dose from a first source and then thesecond detector detects a dose from the second source. However, some ofthe scattered dose from one source will be incident onto the detectorintended for the other source. Therefore when the first detector isbeing readout, the scattered X-rays from the second source will alterthe photodiode signal and hence image that is been read. Consequently itis necessary that during readout, the first detector is insensitive toX-rays from the second source and the second detector is insensitive toX-rays from the first source. This mode of detector operation is calledFrame Transfer. It literally means that during the exposure, the pixeldata is “stored/transferred” onto a storage device within the pixel.After the exposure, the storage device is read and any signal on thephotodiodes from scattered X-rays will not effect the signal beingreadout.

[0093] In the conventional X-ray detector, in which the pixels comprisea TFT and photodiode, the detector is exposed to an X-ray dose and theneach line is readout in turn. The readout process resets the pixels sothat at the end of the readout period, the array is reset. Therefore,after the exposure, the detector is still sensitive to X-rays until theentire array has been read. For this reason, the standard detector doesnot provide Frame Transfer operation.

[0094] A modified readout technique can provide the frame transferoperation, and the timing scheme is shown in FIG. 6. Again, the readoutsequence comprises an exposure (phase I) followed by a series of linereadouts (phase II). The top plot in FIG. 6 shows the X-ray exposuretime. The second plot shows the state of all read out switchingtransistors 30 and the next plot shows the state of all resettransistors 27. The next plot shows two read out pulses for the firstrow, and the last plot shows two read out pulses for the last row.

[0095] In this timing arrangement, during the exposure (phase I), thechange in source voltage of the source follower transistor 21 istransferred immediately to the storage capacitor. For this purpose, allthe read out switching transistors are closed during exposure, so thatthe sampling capacitors will be charged to the source voltage duringexposure.

[0096] Assume that the gate voltage at the end of the exposure is:

V _(G1) =V _(G0) +ΔV _(pd)

[0097] and that the source voltage is:

V _(S1) =V _(bias) +ΔV _(pd)

[0098] where ΔV_(pd) is the change in source follower gate voltageduring an exposure, V_(G0) is the DC voltage on the source follower gateat the start of the exposure and V_(bias) is the quiescent voltage onthe source node. Therefore, the charge stored on the sampling capacitoris:

Q _(C) =C _(S) ×V _(S1) =C _(S)×(V _(bias) +ΔV _(pd))

[0099] After the exposure period, all of the read out switchingtransistors 30 open and the reset transistors close. The source followergate and source nodes are reset to V_(G1) and V_(bias). Since the resettransistors 27 remain on during the readout period, the photodiodecharge remains constant. Therefore, X-rays incident on the detectorafter the exposure time will not alter the signal being readout throughthe sampling capacitors so that Frame Transfer operation is possible.

[0100] During the readout period, each read out switching transistor issequentially addressed. During this time, the sampling capacitor in anaddressed pixel will be charged to the quiescent source voltage(V_(bias)). Therefore, the change in charge during readout is:

ΔQ _(C) =C×ΔV _(pd)

=>ΔQ _(C) =C _(S) /C _(P) ×ΔQ _(pd)

[0101] and this charge is detected by the amplifier.

[0102] Thus, the pixel can be used to provide frame transfer mode ofoperation and maintain pixel gain with a gain of C_(S)/C_(P).Alternatively, the sampling capacitor could have the same value as C_(P)so there would be frame transfer without gain.

[0103] Finally, it is also to be noted again that all of said advantagesare achieved by means of comparatively few additional components in theindividual sensor cells and without taking additional process stepsduring the production. Therefore, the manufacture of such sensorsaccording to the invention is hardly more expensive than the manufactureof sensors commercially available thus far.

1. A sensor with a plurality of sensor elements, each of which includesa radiation-sensitive conversion element which generates an electricsignal in dependence on the incident radiation, and also with means foramplifying the electric signal in each sensor element and a read-outswitching element in each sensor element which is connected to aread-out line in order to read out the electric signal, characterized inthat the means for amplifying include a respective source followertransistor whose gate is connected to the conversion element, whosesource is connected to an active load and to one side of a samplingcapacitor, the other side of the sampling capacitor being connected tothe read-out line via the read-out switching element, and that arespective reset element is connected to the conversion element resetthe conversion element to an initial state.
 2. A sensor as claimed inclaim 1, characterized in that a discharge switching element isconnected parallel to the sampling capacitor.
 3. A sensor as claimed inclaim 1 or 2, characterized in that the active load and/or the read-outswitching element and/or the reset element and/or the dischargeswitching element include transistors.
 4. A sensor as claimed in claim1, 2 or 3, characterized in that the reset element and the dischargeswitching element have a common switching line or separate switchinglines for activating the relevant element.
 5. A sensor as claimed in oneof the claims 1 to 4, characterized in that a plurality of sensorelements have a common switching line for activating their read-outswitching elements, and that these sensor elements also include commonswitching lines or a common switching line for activating their resetelements and/or their discharge switching elements.
 6. A method ofoperating a sensor having a plurality of sensor elements, each of whichincludes a radiation-sensitive conversion element which generates anelectric signal in dependence on the incident radiation, a reset elementwhich resets the conversion element to an initial state, and a sourcefollower transistor whose source is connected to an active load and toone side of a sampling capacitor whose other side is connected, via aread-out switching element, to a read-out line for reading out theelectric signal, wherein: during a measuring and read-out cycle in eachsensor element the conversion element and the sampling capacitor arereset to an initial state during a first phase, a voltage differencewhich is representative of the conversion element in the initial stateis adjusted across the sampling capacitor during a second phase, thevoltage across the sampling capacitor is sustained during a third phasewhile the conversion element is irradiated by means of a radiationsource, and during a fourth phase the voltage difference across thesampling capacitor is adjusted to a value which is representative of theconversion element after the irradiation, the variation of the potentialat the side of the sampling capacitor which is connected to the read-outline being measured as a measure of the radiation incident on theconversion element.
 7. A method as claimed in claim 6, wherein withinthe second phase first a dark current is recorded on the conversionelement in a first sub-phase, and that subsequently there is a secondsub-phase in which a voltage difference is adjusted across the samplingcapacitor, which voltage difference corresponds to a reference state ofthe conversion element after the recording of the dark current, and thatsubsequently in a third sub-phase the conversion element is reset to itsinitial state while the voltage difference across the sampling capacitoris maintained.
 8. A method as claimed in claim 6 or 7, wherein theadjustment of the voltage difference across the sampling capacitor inthe second and the fourth phase takes place by activation of theread-out switching element, and that the read-out switching element isdeactivated in order to sustain the voltage difference.
 9. A method asclaimed in one of the claims 6 to 8, wherein the sampling capacitor isreset by activation of a discharge switching element connected parallelto the sampling capacitor.
 10. A method as claimed in one of the claims6 to 9, wherein a measuring and read-out cycle is controlled for aplurality of sensor elements in common via common switching lines. 11.An X-ray examination apparatus, including an X-ray source for emittingan X-ray beam for irradiating an object so as to form an X-ray image, aswell as a detector for generating an electric image signal from saidX-ray image, characterized in that an X-ray detector includes a sensoras claimed in one of the claims 1 to
 5. 12. A method of operating asensor having a plurality of sensor elements arranged in rows andcolumns, each of which includes a radiation-sensitive conversion elementwhich generates an electric signal in dependence on the incidentradiation, a reset element which resets the conversion element to aninitial state, and a source follower transistor whose source isconnected to an active load and to one side of a sampling capacitorwhose other side is connected, via a read-out switching element, to aread-out line for reading out the electric signal, the methodcomprising: resetting the radiation-sensitive element and charging thesampling capacitor of each pixel to a known voltage; exposing the sensorto radiation, the radiation-sensitive conversion element causing thevoltage on the one side of the sampling capacitor to vary, wherein theread-out switching element is open during the exposure, providing anopen circuit at the other side of the sampling capacitor, therebymaintaining a constant charge on the sampling capacitor; and closing theread out switching elements and charging the sampling capacitor for eachpixel in a row to the voltage on the one side of the sampling capacitor,the amount of charge required being measured.
 13. A method of operatinga sensor having a plurality of sensor elements arranged in rows andcolumns, each of which includes a radiation-sensitive conversion elementwhich generates an electric signal in dependence on the incidentradiation, a reset element which resets the conversion element to aninitial state, and a source follower transistor whose source isconnected to an active load and to one side of a sampling capacitorwhose other side is connected, via a read-out switching element, to aread-out line for reading out the electric signal, the methodcomprising: exposing the sensor to radiation with the read-out switchingelements closed, the radiation-sensitive conversion element causing achange in the voltage on the one side of the sampling capacitor and theread out line holding the other side of the sampling capacitor to aconstant voltage; closing the reset elements to and opening the read outswitching elements, thereby holding the conversion element to a constantstate irrespective of the incident radiation; closing the read outswitching elements of each row in turn and measuring the charge storedon the sampling capacitors for each row in turn.